1. Field of the Invention
The teachings herein relate to fabrication of semiconductors, and in particular, to wafer and stage alignment using photonic devices.
2. Description of the Related Art
Optical metrology is an integral and crucial part of semi-conductor manufacturing. For example, optical metrology provides for characterization of a large number of complex structures and phenomena. Thickness, critical dimension, sidewall angle, doping, chemical properties, RIE lag, etch bias, focus and exposure feedback and feed forward are among the many measurements available today. Benefits of using optical metrology include non-invasive and fast measurement capabilities with relatively low cost of ownership. Several factors are critical to successful use of optical metrology techniques. For example, accurate wafer and stage alignment have to be realized prior to performing any measurements.
As a review, optical metrology generally employs at least one of spectroscopic ellipsometry and reflectometry. In spectroscopic ellipsometry, and with reference to FIG. 1, an incident beam of measuring light 5 illuminates a target 10 at an angle, θ. The angle, θ, is of a value that is other than normal to a surface of the target 10. Interrogation of the target 10 with measuring light 5 results in a spot (not shown). The spot resembles an ellipse. In reflectometry, and with reference to FIG. 2, the incident beam of measuring light 5 illuminates the target 10 normal to the surface of the target 10. In most optical systems, reflectometry results in a circular spot.
The optical systems used in spectroscopic ellipsometry and reflectometry to illuminate the sample and collect the optical spectra generally make use of and control various wavelengths and other parameters to improve measurement results. Regardless of which type of optical system is used, reflected measuring light must be analyzed to determine properties of the target 10. Analysis typically includes use of an optical model specifically created for the target being measured.
One skilled in the art will recognize that accurate determinations of properties of the target 10 require proper alignment of optical elements with the wafer. Accordingly, it is important to provide accurate alignment of the target 10 with a stage used for holding the target 10. Accuracy requirements call for a high level of wafer and stage alignment, as well as control of the flatness (of the stage and target 10).
What are needed are accurate techniques for performing alignment of a target wafer and a stage in an optical metrology system. Preferably, the techniques provide for determining a degree of flatness of the stage and the target wafer.